Hot Rod DeVille Burnt PCB

[Just Mike]

There's no conductivity and both R62 & CR5 read ok. Was this arc caused by a bad tube? Anything else I should troubleshoot?

Thanks,

[glebert]

What components is the arcing between? Also give the schematic or at least what revision of the Deville this is for. I don't think there are big changes over the years, but it is helpful to have all the details.

[Greg Robinson]

Ok, so that's arcing between screen grid resistor and flyback diode. There was probably contamination of the board and/or the amp was run without a load.
There can be more than B+ between those two nodes, so a simple multimeter check for resistance between the two pads is insufficient, you really need at least a megger/insulation resistance meter at 1kV or higher to be sure that there's no longer dielectric breakdown between the pads.Such tests should be done with the output transformer disconnected from circuit.
I would probably completely excise the burnt PCB to virgin FR4, apply corona dope or similar, cut the traces to the diode pads to isolate it, and relocate the diode itself to the tube socket to be sure of the repair.

[drewl]

Is this one of the newer ones?
Last week I had a Hot Rod Deluxe III with a pc board short between the left output tube bias feed and the plate lead.
No visual contamination or evidence of anything that could have caused the short.

[Just Mike]

This is the schematic. Fender_hotrod_deville.pdf

The arc was between CR5 & R62. I also found that R65 was open.

[The Dude]

FWIW: I've seen these types of burns when there are screws, nuts, or other conductive items rolling around in the chassis. So, give the amp a good visual inspection and a shake to make sure nothing falls out or is still inside the amp.

[Just Mike]

Got it up and running but there's a slight fluttering noise. I cleaned the power amp send& return jacks. But more importantly, with new tubes, I can't get the bias below about 72mv at TP 30. Suggestions?

[g1]

Measure bias voltage at negative end of C38. Check on both DC and AC ranges.

[Just Mike]

Negative end of C38 (C-) is -53 VDC. Minimal AC voltage reading here also (my meter says -.1VAC). With tubes in, I'm getting 61-62mv at TP 30 with the pot turned all the way down. BTW, tubes are brand new GT 6l6's.

[g1]

That's just a touch weak, enough to limit the available bias range. Proper -54V there would allow you to bias cooler.
Do you have -74V at the negative end of C43? What is your line voltage coming into the amp?
(also check that no one has altered R77 value)

[Just Mike]

With AC line voltage of 122/123VAC, I'm getting -77 at C43 in standby and -74.5v in On position. R77 is 100k.
Also, with v1,v2 & v3 pulled, I'm still getting crackling sounds.

[Just Mike]

After looking at the schemo and checking component values, I guessed that it could only be a connection problem. So I reflowed the PCB around the power tubes. The amp now biases correctly and is as quiet as a mouse. Thanks for the help!